1. Field of the Invention
The present invention relates to a magnetic toner for developing an electrostatic image, containing spherical ferrite particles.
2. Related Background Art
Dry developing processes hitherto used in image forming processes such as electrophotography and electrostatic recording are chiefly grouped into a process in which a two-component developer is used and a process in which a one-component developer is used.
In the developing process that uses a two-component developer, a mixed developer comprising carrier particles and toner particles is used. There is usually the problem that a mixing ratio of the toner and carrier varies with progress of developing or the image quality of a toner image is lowered because of deterioration or the like of carrier particles.
On the other hand, the developing process that uses a one-component developer contains no carriers, and hence is free from the above problem of the variation of a mixing ratio or the deterioration of carrier particles. Thus, it is an electrostatic-image developing process capable of forming a toner image faithful to an electrostatic image of the toner image and also capable of achieving stable image quality. In particular, a process in which a developer comprising toner particles having magnetic properties is used can often bring about excellent results.
Such a developing process is exemplified by a process proposed in U.S. Pat. No. 3,900,258, in which development is carried out using a magnetic toner having an electrical conductivity. In this developing process, a conductive magnetic developer is supported on a cylindrical conductive sleeve having a magnet in its interior, and this developer is brought into contact with a recording medium having an electrostatic image to carry out development. Here, in a developing section, conductive magnetic toner particles form a conductive path between the surface of the recording medium and the surface of the sleeve, where electric charges are introduced into the conductive magnetic toner particles from the sleeve through the conductive path, and, because of Coulomb force acting between an electrostatic image and conductive magnetic toner particles, the conductive magnetic toner particles adhere to the electrostatic image. The electrostatic image can be thus developed. While the developing process in which a conductive magnetic toner is used is a superior process free of the problems involved in the conventional developing process in which a two-component developer is used, it has the problem that the toner, which is conductive, makes it difficult to electrostatically transfer a toner image from a recording medium to a transfer medium such as plain paper.
As a developing process in which a high-resistivity magnetic toner capable of electrostatic transfer is used, Japanese Patent Laid-open No. 52-94140 disclosed a developing process in which the dielectric polarization of a toner is utilized. Such a process, however, has the problem that the rate of development is fundamentally too low to obtain a satisfactory density in a developed image.
As a developing process in which a high-resistivity magnetic toner is used, a process is known in which magnetic toner particles are triboelectrically charged by friction between magnetic toner particles themselves or friction between magnetic toner particles and a sleeve to carry out development. In such a developing processes, however, the contact between toner particles and a friction member tends to be so few that the triboelectric charging between toner particles may be insufficient.
Japanese Patent Laid-open No. 54-43037 (corresponding to U.S. Pat. No. 4,386,577) discloses a proposal for a novel developing process which is an improvement of a conventional developing process. In this developing process, a magnetic toner is coated on a sleeve in a very small thickness, the resulting magnetic toner layer is triboelectrically charged, and is then brought very close, and also face-to-face without contact, to an electrostatic image in the presence of a magnetic field. The electrostatic image is thus developed. According to this process, a superior image can be obtained because of the advantages that the application of a magnetic toner on a sleeve in a very small thickness increases the opportunity of contact between the sleeve and the toner to enable sufficient triboelectric charging. Since the toner is supported by the action of a magnetic force, and a magnet and the toner is moved in a relative fashion, the agglomeration between toner particles can be released and a sufficient friction can be attained between toner particles and the sleeve. Since the development is carried out while the toner is supported by the action of a magnetic force and the magnetic toner layer is brought face-to-face to an electrostatic image without contact therewith, ground fogging can be prevented.
In recent years, with a rapid progress in copying machines and printers that employ electrophotography and digital latent image techniques, toners are required to have higher performance. Particularly in printers, because of the development of a digital image, it is required as a matter of course that toner images with the same quality can be repeatedly obtained.
Besides characters, printers must be also able to print out images such as graphic images and photographic images. Hence, they are required to have a higher reproducibility of halftone images and fine line images than the conventional printer. In particular, some of recent printers can form an image with 400 dots or more per inch, where a digital latent image on a photosensitive member has become more detailed. Thus, a higher reproducibility of halftone images and fine lines is required in development. In addition, it is increasingly demanded that an image with a high image density and a high image quality must be obtained even in varied environments.
Under the circumstances as stated above, a further improvement is desired in the magnetic toners conventionally used.
In order to obtain a high image density in various environments, it is important to stably keep the amount of triboelectric charge of magnetic toner particles to an appropriate value. In this regard, some methods have been proposed, including, for example, a method in which a compositional improvement is made on a magnetic powder so that the electrical resistance of the magnetic powder can be increased, or the particle surfaces of a magnetic powder are modified so that the water absorption properties (making them more hydrophobic) of the magnetic powder can be improved. This is based on the idea that, with an increase in the electrical resistance or hydrophobicity of a magnetic powder, the charge of a magnetic toner can be more stably retained in the case of a magnetic toner that employs such a magnetic powder.
Among the above proposals, a comparison can be made between the method in which the particle surfaces of a magnetic powder are modified and the method in which the magnetic powder itself is compositionally changed. The former additionally requires the step of surface treatment on the magnetic powder when it is prepared, resulting not only in an increase in cost but also an increase in steps. This produces a possibility that the performance may greatly differ between production lots. From these viewpoints, the latter method in which the magnetic powder itself is compositionally changed can be said to be a better method.
Proposals on the latter method includes those disclosed in Japanese Patent Laid-open No. No. 55-65406 (corresponding to U.S. Pat. No. 4,282,302) and No. 57-77031.
The Japanese Patent Laid-open No. 55-65406 discloses a magnetic toner employing spinel type ferrite particles containing a compound of a divalent metal selected from Mn, Ni, Mg, Cu, Zn and Cd. The Japanese Patent Laid-open No. 57-77031 discloses a process for preparing a black, cubic spinel type iron oxide comprising a solide solution with zinc, which is a wet method, particularly characterized in that a zinc ion is added in the course of oxidation of a ferrous salt solution.
The magnetic toner in which the magnetic powder as in the above two proposals is used undoubtedly exhibits a higher performance than conventional toners in view of the advantages that the charge of the toner can be kept in an appropriate amount and in a more stable and the image density can be made higher. However, black spots of the toner may be formed around an image and hence can not answer the new demand for a higher reproducibility of halftone dots or fine lines. This is for one thing ascribable to its coercive force Hc which is not less than 100 Oe.
On the other hand, with regard to an attempt to enhance the reproducibility of halftone dots or fine lines, Japanese Patent Laid-open No. 59-220747 discloses a proposal that a magnetic toner can be less agglomerated, with a high fluidity, and a sharp and excellent toner image can be obtained when a magnetic material used in the toner image has a small coercive force. In this proposal, however, it is proposed to use iron or an iron alloy as a magnetic powder having a small coercive force. The iron or iron alloy has, for example, an electrical resistivity of 10.sup.-5 .OMEGA.cm, which is much lower than ferrite, and hence is not preferable when one takes into account that the triboelectric properties of a magnetic toner must be stabilized.